Recently there has been a lot of patents filed where the production and use of a photocatalytic coating containing a photocatalyst material such as titania (TiO2) is described. The said photocatalytic coating find its application in various field of technology, such as in the anti-fouling area [US 6013372] and in antibacterial applications [JP 11047610].
When a photocatalyst, such as titanium dioxide, is subjected to photoexcitation by ultraviolet light (UV), water is chemisorbed on the surface of the titanium ion in the form of hydroxyl groups (OH—) as a result of the photocatalytic action. As a result, the surface of the material coated with TiO2 becomes very much hydrophilic. Due to said properties, a transparent film containing photocatalytic layer can be used as anti fogging material [US 6013372].
Another suggestion on the mechanism of photocatalytic action is the formation of activated oxygen from water or oxygen in the air when TiO2 captures UV light. This process is similar to photosynthesis, in which chlorophyll captures sunlight to turn water and carbon-dioxide into oxygen and glucose. This activated oxygen has the ability to decompose organic substances and micro-organism which are present near to the surface of the material that is coated with TiO2. As a result, the surface which is covered by photocatalytic particles has antibacterial or disinfecting properties [JP 11047610]. Since the activated oxygen has the affinity to decompose organic substances, it can also be applied in a deodorising agent. By coating an aqueous dispersion containing photocatalytic particles and binder on a base paper, one can produce deodorising paper suitable to be used in air cleaner or for other gas streams [JP 11279446, JP 11117196].
The most important property of a photocatalyst material is that upon light radiation with an energy that is higher than the valence band of the photocatalyst material, an electron and an electron-hole will be generated by excitation. The excited electron reacts with oxygen and will decompose the surrounding polymers and other organic materials that have a lower bonding energy than the valence energy of the photocatalyst material by oxidation reaction. AB an example, when TiO2 is irradiated with UV light, decomposition of the surrounding polyolefin resin or cellulose will occur. However poly-fluoro-carbon resin will not be decomposed since the bonding energy of C-F is higher than the valence band of TiO2. It is thus of importance to select a suitable material for being used as binder for the photocatalytic layer. Examples of suitable binders for TiO2 are organic polymers containing fluorocarbon and/or inorganic materials like silica oxide.
The conventional method to produce a sheet containing photocatalysts is by coating an aqueous dispersion containing photocatalyst particles, inorganic binder, like colloidal silica or silicone resin, and organic binder, such as a thermoplastic polymer having a high deterioration resistance—polytetrafluoroethylene—on a film or paper. After drying the coating solution, the coated paper is ready for cutting and packaging. This method is suggested among others in JP 11279446, JP 11117196, JP 10128125.
In JP 11117194, it was proposed to coat the paper with a UV absorbing layer, followed by coating said layer with a photocatalyst containing layer that contains expandable microcapsules, and then drying the coating by heating. Due to the heat, the microcapsules will expand and create specific desirable surface areas.
Generally, in order to manufacture photocatalytic film sheets on a support, the layer comprising a polymer with a lower bonding energy than the valence band of the photocatalytic material, should be protected from a direct contact with the photocatalytic material. For this purpose, a protective layer comprising polymer compounds with a high bonding energy than that of the photocatalytic layer, are applied on the support. Thereafter a dispersion solution containing photocatalyst material, binder and inorganic material are coated on the surface of said protective layer. The common techniques to apply said protective and photocatalyst layers on the support are the coating or spraying methodologies such as dip coating, spin coating, wire bar coating, blade coating, roller-coating, spray coating [JP 07171408].
The combination of the coating, drying and optional baking processes, which is described in JP 11047610, is known to be slow. As a result, the production speed becomes low. The more drying and baling steps needed, the slower the production speed will be. Another disadvantage of this process is that it costs relatively a lot of energy because first of all the photocatalytic particles have to be dispersed uniformly in the coating solution and secondly evaporating the solvent after coating in the drying or baling step.
It would be desirable if the photocatalytic film sheet could be manufactured at a high production speed and without any necessity for drying or baking processes.